CS162 Operating Systems and Systems Programming Lecture 23 Remote Procedure Call November 27, 2013 Anthony D. Joseph and John Canny

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Goals for Today Remote Procedure Call Examples using RPC –Distributed File Systems –World-Wide Web Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne, notes by Joseph and Kubiatowicz.

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Distributed Systems – Message Passing Distributed systems use a variety of messaging frameworks to communicate: –e.g. the protocols for TCP: connecting, flow control, loss… –2PC for transaction processing –HTTP GET and POST –UDP messages for MS SQL Server (last time) Disadvantages of message passing: –Complex, stateful protocols, versions, feature creep –Need error recovery, data protection, etc. –Ad-hoc checks for message integrity –Resources consumed on server between messages (DoS risk) –Need to program for different OSes, target languages,… Want a higher-level abstraction that addresses these issues, but whose effects are application-specific

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Remote Procedure Call Another option: Remote Procedure Call (RPC) –Looks like a local procedure call on client: file.read(1024); –Translated automatically into a procedure call on remote machine (server) Implementation: –Uses request/response message passing “under the covers” –Deals with many of the generic challenges of protocols that use message passing – may even be “transactional” - but usually not. –Allows the programmer to focus on the message effects: as though the procedure were executed on the server.

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 RPC Details Client and server use “stubs” to glue pieces together –Client stub is responsible for “marshalling” arguments and “unmarshalling” the return values –Server-side stub is responsible for “unmarshalling” arguments and “marshalling” the return values Marshalling involves (depending on system) converting values to a canonical form, serializing objects, copying arguments passed by reference, etc. –Needs to account for cross-language and cross-platform issues Technique: compiler generated stubs –Input: interface definition language (IDL) »Contains, among other things, types of arguments/return –Output: stub code in the appropriate source language

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 RPC Information Flow Client (caller) Server (callee) Packet Handler Packet Handler call return send receive send receive return call Network Client Stub bundle args bundle ret vals unbundle ret vals Server Stub unbundle args Machine A Machine B

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 RPC Binding How does client know which machine to send RPC? –Need to translate name of remote service into network endpoint (e.g., host:port) –Binding: the process of converting a user-visible name into a network endpoint »This is another word for “naming” at network level »Static: fixed at compile time »Dynamic: performed at runtime Dynamic Binding –Most RPC systems use dynamic binding via name service –Why dynamic binding? »Access control: check who is permitted to access service »Fail-over: If server fails, use a different one Object registry (if used) –Contains remote object names and client stub code –Allows dynamic loading of remote object stub

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Cross-Domain Communication/Location Transparency How do address spaces communicate with one another? –Shared Memory with Semaphores, monitors, etc… –File System –Pipes (1-way communication) –“Remote” procedure call (2-way communication) RPC’s can be used to communicate between address spaces on different machines or the same machine –Services can be run wherever it’s most appropriate –Access to local and remote services looks the same Examples of modern RPC systems: –ONC/RPC (originally SUN RPC) in Linux, Windows,… –DCE/RPC (Distributed Computing Environment/RPC) –MSRPC: Microsoft version of DCE/RPC –RMI (Java Remote Method Invocation)

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Microkernel Operating Systems Example: split kernel into application-level servers using RPC –File system looks remote, even though on same machine App file system Windowing Networking VM Threads App Monolithic Structure App File sys windows RPC address spaces threads Microkernel Structure

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Microkernel Operating Systems Why split the OS into separate domains? –Fault isolation: bugs are more isolated (build a firewall) –Enforces modularity: allows incremental upgrades of pieces of software (client or server) –Location transparent: service can be local or remote »For example in the X windowing system: Each X client can be on a separate machine from X server; Neither has to run on the machine with the frame buffer App file system Windowing Networking VM Threads App Monolithic Structure App File sys windows RPC address spaces threads Microkernel Structure

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Problems with RPC Handling failures –Different failure modes in distributed system than on a single machine –Without RPC a failure within a procedure call usually meant whole application would crash/die –With RPC a failure within a procedure call means remote machine crashed, but local one could continue working –Answer? Distributed transactions can help Performance –Cost of Procedure call « same-machine RPC « network RPC –Means programmers must be aware they are using RPC (so much for transparency!) »Caching can help, but may make failure handling even more complex

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Distributed File Systems Distributed File System: –Transparent access to files stored on a remote disk –Transparent concurrency: All clients have the same view of the state of the file system. –Failure transparency The client and client programs should operate correctly after a server failure. –Replication transparency To support scalability, we may wish to replicate files across multiple servers. Clients should be unaware of this. –Migration transparency Files should be able to move around without the client's knowledge. Network Read File Data Client Server

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Distributed File Systems Naming choices (always an issue): –Hostname:localname: Name files explicitly »No location or migration transparency –Mounting of remote file systems »System manager mounts remote file system by giving name and local mount point »Transparent to user: all reads and writes look like local reads and writes to user e.g. /users/sue/foo  /sue/foo on server –A single, global name space: every file in the world has unique name »Location Transparency: servers can change and files can move without involving user mount coeus:/sue mount adj:/prog mount adj:/jane

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Simple Distributed File System EVERY read and write gets forwarded to server Advantage: Server provides completely consistent view of file system to multiple clients Problems? Performance! –Going over network is slower than going to local memory –Server can be a bottleneck Client Server Read (RPC) Return (Data) Client Write (RPC) ACK cache

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Failures What if server crashes? Can client wait until server comes back up and continue as before? –Any data in server memory but not on disk can be lost –Shared state across RPC: What if server crashes after seek? Then, when client does “read”, it will fail –Message retries: suppose server crashes after it does UNIX “rm foo”, but before acknowledgment? »Message system will retry: send it again »How does it know not to delete it again? (could solve with two- phase commit protocol, but NFS takes a more ad hoc approach) Crash!

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Stateless Protocol Stateless protocol: A protocol in which all information required to process a request is passed with request –Server keeps no state about client, except as hints to help improve performance (e.g. a cache) –Thus, if server crashes and restarted, requests can continue where left off (in many cases) What if client crashes? –Might lose modified data in client cache Examples: –HTTP –REST (Representational State Transfer) Stateful –SOAP (Simple Object Access Protocol) - usually over HTTP! Crash!

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Network File System (NFS) Three Layers for NFS system –UNIX file-system interface: open, read, write, close calls + file descriptors –VFS layer: distinguishes local from remote files »Calls the NFS protocol procedures for remote requests –NFS service layer: bottom layer of the architecture »Implements the NFS protocol

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Schematic View of NFS Architecture

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Network File System (NFS) NFS Protocol: RPC for file operations on server –Reading/searching a directory –Manipulating links and directories –Accessing file attributes/reading and writing files Write-through caching: Modified data committed to server’s disk before results are returned to the client –Lose some of the advantages of caching –Time to perform write() can be long –Need some mechanism for readers to eventually notice changes! (more on this later)

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 NFS Continued NFS servers are stateless; each request provides all arguments require for execution –E.g. reads include information for entire operation, such as ReadAt(inumber,position), not Read(openfile) –No need to perform network open() or close() on file – each operation stands on its own Idempotent: Performing requests multiple times has same effect as performing it exactly once –Example: Server crashes between disk I/O and message send, client resend read, server does operation again –Example: Read and write file blocks: just re-read or re-write file block – no side effects –Example: What about “remove”? NFS does operation twice and second time returns an advisory error

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 NFS Continued Failure Model: Transparent to client system –Is this a good idea? What if you are in the middle of reading a file and server crashes? –Options (NFS Provides both): »Hang until server comes back up (next week?) »Return an error. (Of course, most applications don’t know they are talking over network)

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 NFS protocol: weak consistency –Client polls server periodically to check for changes »Polls server if data hasn’t been checked in last 3-30 seconds (exact timeout it tunable parameter). »Thus, when file is changed on one client, server is notified, but other clients use old version of file until timeout. What if multiple clients write to same file? »In NFS, can get either version (or parts of both) »Completely arbitrary! NFS Cache consistency cache F1:V2 Server Write (RPC) ACK Client cache Client cache F1:V1 F1:V2 F1 still ok? No: (F1:V2)

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 NFS Pros and Cons NFS Pros: –Simple, Highly portable NFS Cons: –Sometimes inconsistent! –Doesn’t scale to large # clients »Must keep checking to see if caches out of date »Server becomes bottleneck due to polling traffic

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Andrew File System Andrew File System (AFS, late 80’s)  DCE DFS (commercial product) Callbacks: Server records who has copy of file –On changes, server immediately tells all with old copy –No polling bandwidth (continuous checking) needed Write through on close –Changes not propagated to server until close() –Session semantics: updates visible to other clients only after the file is closed »As a result, do not get partial writes: all or nothing! »Although, for processes on local machine, updates visible immediately to other programs who have file open In AFS, everyone who has file open sees old version –Don’t get newer versions until reopen file

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Andrew File System (con’t) Data cached on local disk of client as well as memory –On open with a cache miss (file not on local disk): »Get file from server, set up callback with server –On write followed by close: »Send copy to server; tells all clients with copies to fetch new version from server on next open (using callbacks) What if server crashes? Lose all callback state! –Reconstruct callback information from client: go ask everyone “who has which files cached?”

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Andrew File System (con’t) AFS Pro: Relative to NFS, less server load: –Disk as cache  more files can be cached locally –Callbacks  server not involved if file is read-only For both AFS and NFS: central server is bottleneck! –Performance: all writes  server, cache misses  server –Availability: Server is single point of failure –Cost: server machine’s high cost relative to workstation

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Administrivia MIDTERM II 5:30-7pm in 145 Dwinelle (A-L) and 2060 Valley LSB (M-Z) –Review: 306 Soda 7-9pm, Sunday Dec 1 –Covers Lectures #14-24, projects, and readings –One sheet of notes, both sides Project 4 Initial Design Due Monday

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall min Break

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Q1: True _ False _ RPC requires special networking support and functionality Q2: True _ False _ The client and server for RPC must use the same hardware architecture (e.g., little endian) Q3: True _ False _ Local procedure call << same-machine RPC << remote machine RPC Q4: True _ False _ NFS provides weak client-server data consistency Quiz 23.1: RPC and NFS

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Q1: True _ False _ RPC requires special networking support and functionality Q2: True _ False _ The client and server for RPC must use the same hardware architecture (e.g., little endian) Q3: True _ False _ Local procedure call << same-machine RPC << remote machine RPC Q4: True _ False _ NFS provides weak client-server data consistency Quiz 23.1: RPC and NFS X X X X

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Distributed Object-Oriented Systems Distributed systems, like any complex software benefit from careful software architecture, especially object-oriented programming. Major efforts were devoted to OOP distributed systems architectures: CORBA (Common Object Request Broker Architecture) DCOM (Distributed Component Object Model) from MS, which drew heavily from the open system DCE/DFS These systems use remote methods, and add object proxying and even garbage collection.

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Internet-Scale Distributed Computing CORBA and DCOM were robust, powerful RPC-based distributed object systems. They were supposed to become the substrate for internet-scale distributed computing. What happened? (they didn’t) From last time: –Morris worm –Code Red –Slammer …………………. which led to… Ubiquitous firewalls, packet filters etc., across the internet. HTTP (port 80) was the only reliable route to a remote host

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Internet-Scale Distributed Computing One approach is to tunnel other types of payload (other than HTTP) through port 80, and demultiplex at the server. Usually, but not always, this works. Instead many systems have used HTTP directly as a high- level transport for RPC. A cluster of technologies have developed around data messaging, RPC and distributed objects over HTTP: –SOAP/WSDL –REST –and enabled by XML and JSON (JavaScript Object Notation)

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 WWW- SOAP RPC SOAP covers the following four main areas: A message format for one-way communication describing how a message can be packed into an XML document. A description of how a SOAP message should be transported using HTTP (for Web-based interaction) or SMTP (for -based interaction). A set of rules that must be followed when processing a SOAP message and a simple classification of the entities involved in processing a SOAP message. A set of conventions on how to turn an RPC call into a SOAP message and back.

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Soap Message Typically an XML element containing header and body elements

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 SOAP RPC SOAP RPC messages typically encode arguments that are presented to the calling program as parameters and return values:

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Soap RPC

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Soap Response

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 WSDL Is “Web Services Description Language” an XML format for specifying metadata about a SOAP protocol. WSDL is used to describe precisely what a service does, i.e., the operations the service provides, where it resides, i.e., details of the protocol specific address, e.g., a URL, and how to invoke it, i.e., details of the data formats and protocols

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 WSDL Service Implementation section  Service Interface section 

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 REST – “post RPC” REpresentation State Transfer Stateless Client/Server Protocol: Principles 1.Each message in the protocol contains all the information needed by the receiver to understand and/or process it. This constraint attempts to “keep things simple” and avoid needless complexity 2.Set of Uniquely Addressable Resources –“Everything is a Resource” in a RESTful system –Requires universal syntax for resource identification (e.g. URI)

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 REST 3.Set of Well-Defined Operations that can be applied to all resources –In context of HTTP, the primary methods are –POST, GET, PUT, DELETE –these are similar (but not exactly) to the database notion of –CRUD (Create, Read, Update, Delete) 4.The use of Hypermedia both for Application Information and State Transitions –Resources are typically stored in a structured data format that supports hypermedia links, such as XHTML or XML

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 REST Idempotency: repeated application of the operation does not change the state of the target

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 REST example Jane female New York City, NY, USA This documentation is a representation used for the User resource It might live at If a user needs information about Jane, they GET this resource If they need to modify it, they GET it, modify it, and PUT it back The href to the Location resource allows savvy clients to gain access to its information with another simple GET request Implication: Clients cannot be “thin”; need to understand resource formats

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 REST vs. RPC In RPC systems, the design emphasis is on verbs What operations can I invoke on a system? getUser(), addUser(), removeUser(), updateUser(), getLocation(), updateLocation(), listUsers(), listLocations(), etc. In REST systems, the design emphasis is on nouns User, Location In REST, you would define XML representations for these resources and then apply the standard methods to them

/27/2013 Anthony D. Joseph and John Canny CS162 ©UCB Fall 2013 Conclusion Remote Procedure Call (RPC): Call procedure on remote machine –Provides same interface as procedure –Automatic packing and unpacking of arguments without user programming (in stub) Distributed File System: –Transparent access to files stored on a remote disk »NFS uses caching for performance SOAP and WSDL: –An RPC protocol and an RPC description format REST: –Simplicity of RPC without any state and without “verbs”